A Schottky gate depletion-mode field effect device is sensitive to ESD damage due to its delicate metallic gate structure (0.5 μm or smaller metal gate length). Unlike CMOS silicon or bipolar transistor processes, there is no robust P-N junction diode available in a high-electron-mobility transistor (HEMT) process to form a compact ESD protection diode. A HEMT, also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region (as is generally the case for MOSFET). There are several versions of the HEMT, e.g., pseudomorphic HEMT (pHEMT), metamorphic HEMT (mHEMT), induced HEMT, etc. Several large Schottky diodes formed with the gate of the pHEMT device have to be cascaded in series to adequately protect the active HEMT circuits. These multiple Schottky diodes consume a large area in an expensive GaAs integrated circuit die.
A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode. A normal diode will have a voltage drop between 0.6 to 1.7 volts, while a Schottky diode voltage drop is usually between 0.15 and 0.45 volts. This lower voltage drop provides better system efficiency and higher switching speed. In a Schottky diode, a semiconductor-metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. An N-type semiconductor acts as the cathode and the metal side acts as the anode of the Schottky diode. This Schottky barrier results in both a low forward voltage drop and very fast switching. ESD protection can be provided by cascading several large Schottky diodes. Due to the Schottky diode's low turn on voltage, several stacked diodes are required to handle the operating voltage, and a large area is required to handle the current. Thus using a Schottky diode configuration requires a large die area.